Final answer:
Red-colored letters appear raised up under a glass slab due to the refraction of light, where light bends as it passes from one medium to another, with red light being refracted the least. Option B is the correct answer.
Step-by-step explanation:
When a plane glass slab is placed over letters of different colors, the red-colored letters appear to be more raised up due to the effect of refraction of light. Refraction occurs when light passes from one medium to another, changing speed and bending as a result. Since different colors of light have different wavelengths, they refract at slightly different angles. Red light, having the longest wavelength of visible light, is refracted less than other colors when passing through the glass to the air and then to our eyes. It, therefore, appears to be at a higher position relative to other colors, which are refracted more and seem to be lower.
This phenomenon can also be related to dispersion, which occurs when light separates into its spectral components because different wavelengths of light are refracted by different amounts as they pass through a prism. However, since the question is specifically about the appearance of red-colored letters being raised, the correct explanation aligns with the principles of refraction rather than dispersion, interference, or diffraction of light. Thus, in the context of the question, the correct option is (b) Refraction of light.